The detection of infectious diseases is often accomplished by use of tests that monitor immunological responses. Many times, however such tests are cumbersome and frequently yield inconsistent results. In addition, the absence of sophisticated laboratory equipment often reduces the availability of testing to individuals living in underdeveloped areas where the occurrence of infectious disease may be disproportionately high. Accurate diagnosis and detection of infectious disease is important not only for treatment purposes, but also for the prevention of occurrence and dissemination of disease. The need for sensitive and accurate detection methods has become particularly pronounced recently especially in light of the increase in infections such as those caused by mycobacteria.
Mycobacterial infections often manifest as diseases such as tuberculosis. Human infections caused by mycobacteria have been widespread since ancient times, and tuberculosis remains a leading cause of death today. Although the incidence of the disease declined in parallel with advancing standards of living since at least the mid-nineteenth century, mycobacterial diseases still constitute a leading cause of morbidity and mortality in countries with limited medical resources and can cause overwhelming, disseminated disease in immunocompromised patients. In spite of the efforts of numerous health organizations worldwide, the eradication of mycobacterial diseases has never been achieved, nor is eradication imminent. Nearly one third of the world's population is infected with M. tuberculosis complex, commonly referred to as tuberculosis (TB), with approximately 8 million new cases and 3 million deaths attributable to TB yearly.
After decades of decline, TB is on the rise. In the United States, up to 10 million individuals are believed to be infected. Almost 28,000 new cases were reported in 1990, a 9.4 percent increase over 1989. A sixteen percent increase was observed from 1985 to 1990. Overcrowded living conditions and shared air spaces are especially conducive to the spread of TB, contributing to the increase in instances that have been observed in the U.S. in prison inmates and among the homeless in larger cities.
Approximately half of all patients with acquired immune deficiency syndrome (AIDS) will acquire a mycobacterial infection, with TB being an especially devastating complication. AIDS patients are at higher risks of developing clinical TB and anti-TB treatment seems to be less effective than in non-AIDS patients. Consequently, the infection often progresses to a fatal disseminated disease.
Mycobacteria other than M. tuberculosis are increasingly found in opportunistic infections that plague the AIDS patient. Organisms from the M. avium-intracellulare complex (MAC), especially serotypes four and eight, account for 68% of the mycobacterial isolates from AIDS patients. Enormous numbers of MAC are found (up to 1010 acid-fast bacilli per gram of tissue) and, consequently the prognosis for the infected AIDS patient is poor.
The World Health Organization (WHO) continues to encourage the battle against TB, recommending prevention initiatives such as the “Expanded Program on Immunization” (EPI), and therapeutic compliance initiatives such as “Directly Observed Treatment Short-Course” (DOTS). For the eradication of TB, diagnosis, treatment, and prevention are equally important. Rapid detection of active TB patients will lead to early treatment by which about 90% cure is expected. Therefore, early diagnosis is critical for the battle against TB. In addition, therapeutic compliance will ensure not only elimination of infection, but also reduction in the emergence of drug-resistance strains.
The emergence of drug-resistant M. tuberculosis is an extremely disturbing phenomenon. The rate of new TB cases proven resistant to at least one standard drug increased from 10 percent in the early 1980's to 23 percent in 1991. Compliance with therapeutic regimens, therefore, is also a crucial component in efforts to eliminate TB and prevent the emergence of drug-resistant strains.
Although over 37 species of mycobacteria have been identified, more than 95% of all human infections are caused by six species of mycobacteria: M. tuberculosis, M. avium-intracellulare, M. kansasii, M. fortuitum, M. chelonae, and M. leprae. The most prevalent mycobacterial disease in humans is tuberculosis (TB) which is caused by mycobacterial species comprising M. tuberculosis, M. bovis, or M. africanum (Merck Manual 1992). Infection is typically initiated by the inhalation of infectious particles which are able to reach the terminal pathways in lungs. Following engulfment by alveolar macrophages, the bacilli are able to replicate freely, with eventual destruction of the phagocytic cells. A cascade effect ensues wherein destruction of the phagocytic cells causes additional macrophages and lymphocytes to migrate to the site of infection, where they too are ultimately eliminated. The disease is further disseminated during the initial stages by the infected macrophages which travel to local lymph nodes, as well as into the blood stream and other tissues such as the bone marrow, spleen, kidneys, bone and central nervous system. (See Murray et al. Medical Microbiology, The C.V. Mosby Company 219-230 (1990)).
There is still no clear understanding of the factors which contribute to the virulence of mycobacteria. Many investigators have implicated lipids of the cell wall and bacterial surface as contributors to colony morphology and virulence. Evidence suggests that C-mycosides, on the surface of certain mycobacterial cells, are important in facilitating survival of the organism within macrophages. Trehalose 6,6′ dimycolate, a cord factor, has been implicated for other mycobacteria.
The interrelationship of colony morphology and virulence is particularly pronounced in M. Avium. M. avium bacilli occur in several distinct colony forms. Bacilli which grow as transparent or rough colonies on conventional laboratory media are able to multiply within macrophages in tissue culture, are virulent when injected into susceptible mice, and are resistant to antibiotics. Rough or transparent bacilli which are maintained on laboratory culture media often spontaneously assume an opaque colony morphology at which time they fail to grow in macrophages, are avirulent in mice, and are highly susceptible to antibiotics. The differences in colony morphology between the transparent, rough and opaque strains of M. avium are almost certainly due to the presence of a glycolipid coating on the surface of transparent and rough organisms which acts as a protective capsule. This capsule, or coating, is composed primarily of C-mycosides which apparently shield the virulent M. avium organisms from lysosomal enzymes and antibiotics. By contrast, the non-virulent opaque forms of M. avium have very little C-mycoside on their surface. Both resistance to antibiotics and resistance to killing by macrophages have been attributed to the glycolipid barrier on the surface of M. avium. 
Diagnosis of mycobacterial infection is confirmed by the isolation and identification of the pathogen, although conventional diagnosis is based on sputum smears, chest X-ray examination (CXR), and clinical symptoms. Isolation of mycobacteria on a medium takes as long a time as four to eight weeks. Species identification takes a further two weeks. There are several other techniques for detecting mycobacteria such as the polymerase chain reaction (PCR), mycobacterium tuberculosis direct test, or amplified mycobacterium tuberculosis direct test (MTD), and detection assays that utilize radioactive labels.
One diagnostic test that is widely used for detecting infections caused by M. tuberculosis is the tuberculin skin test. Although numerous versions of the skin test are available, typically one of two preparations of tuberculin antigens are used: old tuberculin (OT), or purified protein derivative (PPD). The antigen preparation is either injected into the skin intradermally, or is topically applied and is then invasively transported into the skin with the use of a multiprong inoculator (Tine test). Several problems exist with the skin test diagnosis method. For example, the Tine test is not generally recommended because the amount of antigen injected into the intradermal layer cannot be accurately controlled. (See Murray et al. Medical Microbiology, The C.V. Mosby Company 219-230 (1990)).
Although tuberculin skin tests are widely used, they typically require 2-3 days to generate results, and many times, the results are inaccurate as false positives are sometimes seen in subjects who have been exposed to mycobacteria but are healthy. In addition, instances of mis-diagnosis are frequent since a positive result is not observed only in active TB patients, but also in BCG-vaccinated persons and those who had been infected with mycobacteria but have not developed the disease. It is hard therefore, to distinguish active TB patients from the others, such as household TB contacts, by the tuberculin skin test. Additionally, the tuberculin test often produces a cross-reaction in those individuals who were infected with mycobacteria other than M. tuberculosis (MOTT). Diagnosis using the skin tests currently available is frequently subject to error and inaccuracies.
What is needed are effective tests for detecting the presence of mycobacterial infection. In particular a test that does not require the invasion of the skin surface of the tested person would minimize the exposure of the health care professional administering the test to the bodily fluids of the tested person and lessen the risk of transmission of other infectious agents that may be present in the tested person. In addition, a test that is easily administered and has an easily determined positive or negative outcome is essential when monitoring compliance with a therapeutic regimen for highly infectious diseases such as tuberculosis, particularly in individuals such as homeless persons, prison inmates, schoolchildren and senior citizens.
What is also needed are inexpensive and accurate methods for distinguishing between persons who have active disease states and those persons who have only been immunologically exposed to infectious agents, (such as those persons previously infected with a mycobacterium) but are without active disease, or those persons who have been vaccinated with BCG. Additionally, there is no known method for monitoring the effects of drug therapy in persons infected with a mycobacterium, such as tests that can distinguish between active tuberculosis and other stages of healing or prior exposure. Furthermore, what is also needed is a test that can be easily administered to children, who are especially afraid of currently used skin tests that involve needles or puncturing the skin. Such tests are particularly desirable for monitoring patients particularly AIDS patients who are highly susceptible to mycobacterial infection. In addition, tests that are easily administered and have an easily determined positive or negative outcome are essential when monitoring a disease such as tuberculosis in homeless persons or prison inmates.